We have previously demonstrated that at temperatures where cell growth occurs (Tg) membrane lipids form a unique unilamellar structure whose properties depend only on the lipid composition. This structure is a critical state in which the lipid molecules undergo relatively large fluctuations of molecular area in the plane of the membrane at Tg. This structure is ubiquitous in cell membranes because it forms with lipids from both prokaryotic and eukaryotic cells whose Tgs span virtually the entire range of temperatures for sustaining life on Earth. We have also shown that when the lipid composition becomes inappropriate for a specific Tg as in Alzheimers disease, and metachromatic leukodystrophy, this unique unilamellar lipid structure degenerates to another lipid state that does not support cell growth; relatively small changes in composition are sufficient to cause degeneration of the membrane. The importance of maintaining this critical structure has been demonstrated in the development of sea urchin embryos, which may grow over a range of temperatures; changes in the incubation temperature result in small lipid composition modifications that are essential for the formation of the critical lipid state structure in the nascent membranes. It appears that metabolism maintains membrane lipids in a state that is particularly sensitive to changes in temperature and composition, and where reaction rates are easily modulated by small perturbations of membrane structure. - membranes, phospholipids, Alzheimer's disease, metachromatic leukodystrophy, bilayer structure, critical state